Uninterruptible power supplies or systems (commonly referred to as UPS) are used to provide back-up power to critical loads such as computer systems where loss of line power can result in the interruption of programs and loss of valuable data. A typical UPS includes a battery or a series of batteries interfaced through an inverter to the AC output line. When a fault occurs in the input AC power, the inverter is controlled to provide power from the battery to the AC output line at the same frequency and with substantially the same waveform as the normal input AC power.
Although there are various basic designs for UPS which utilize a battery as the backup power source, in many if not most types of UPS a large capacitor (or capacitors) is connected across the DC bus lines to which the battery and inverter are connected. The capacitor, which may have a large capacitance value, serves to filter out peaks in the voltage on the DC bus lines during charging and discharging, and also helps to filter out high frequency components that may appear on the DC bus lines.
It is sometimes desirable to be able to disconnect the battery from the DC bus lines in the UPS for substantial periods of time, and then reconnect the battery. For example, during transportation and long-term storage of a UPS, it is usually desirable to disconnect the batteries from the rest of the UPS circuitry to minimize drain on the batteries. The batteries may be disconnected from the DC bus lines using various devices, including switches or power plug connectors.
One problem which has been observed upon reconnection of batteries to the DC bus lines is that a large surge current is commonly drawn from the batteries to charge up the capacitor(s) connected across the DC bus lines. When a low impedance source, such as a battery, is connected to a low impedance load, such as a large capacitor with no charge on it, the initial current flow is limited only by the incidental impedances that may exist in the circuit. If, for example, the battery source impedance is on the order of 7 milliohms, the capacitor or capacitor bank has an effective resistance of 10 milliohms, the connectors between the battery and the capacitor have a very low resistance, and the battery has an output voltage of 48 volts, the initial peak current upon connection of the battery to the uncharged capacitors would theoretically be 2800 amperes. A surge current of this magnitude is potentially very detrimental to capacitor life, and can be sufficient to cause immediate capacitor failure. Furthermore, as the connecting switch is closed or the connectors are brought together, sparking of the switch contacts or the mating connectors can occur, which can be damaging to the connectors.
Conversely, when batteries which have been connected to the DC bus lines are disconnected, the charged capacitors connected to the DC bus lines can retain charge for a relatively long period of time, dependent on the internal leakage of the capacitors and any load that may be present on the DC bus lines. To minimize any safety hazards that this capacitor charge will present, it is generally necessary that provision be made to bleed down the voltage across the capacitors to an acceptable level within a prescribed period of time. Generally, the capacitors are considered to be safely discharged when the stored energy is reduced below 20 joules within 5 minutes. The voltage remaining on the capacitors is determined from the expression E=1/2 CV.sup.2, where E is energy in joules (e.g. 20 joules), C is the value of the capacitance of the capacitors, and V is the voltage on the capacitors. The conventional way in which this bleed-down is accomplished is by the addition of a bleed resistor connected across the capacitors. Because this bleed resistor is typically always connected across the capacitors, and thus across the DC bus lines, it is always dissipating energy. Thus, even if the UPS is in a standby mode, the resistor still bleeds down the capacitor and the battery.